1. Field of the Invention
The present invention relates to a semiconductor package, such as a BGA package, and a production method thereof and, more particularly, to a BGA package with excellent moisture resistance and a production method thereof.
2. Related Background Art
BGA (Ball Grid Array) packages have been and are used as LSI packages adapted for increase in integration and speed of LSI devices.
FIG. 1 is a schematic sectional view for explaining a typical example of the BGA package. This BGA package is a BGA package of an OMPAC (Over Molded Pad Array Carrier) type. In this example, solder bumps 13 of a ball shape are arrayed on the bottom surface of substrate 1, and a semiconductor chip 6 is mounted on the top surface of the substrate 1 and wire-bonded by Au wires. The semiconductor chip 6 and bonding wires 7, placed on the top surface of the substrate 1, are encapsulated by single side molding with encapsulating resin 9.
This BGA package is drawing attention as a high-density mounting technique for reasons that the structure is simple and that the ball pitch can be set wide because of the solder bumps 13 provided on the bottom surface of substrate, thereby facilitating mounting of substrate. On the other hand, it is pointed out that it has the following problems; the encapsulating resin 9 molded shrinks to induce a warp of the package itself; cost is high, because an outer frame of the substrate 1, which is used as a carry portion, becomes unnecessary after completion of the solder bumps and is thus cut away.
For solving these problems, there are suggestions to suggest L-BGA (Lead frame supported BGA) packages using a lead frame as a carrier frame. These L-BGA packages have excellent reliability, because the encapsulating resin completely covers the semiconductor device. In addition, the appearance of the packages is also good, because the resin is densely molded in a mold. Therefore, this method is widely used at present. In recent years, there are demands for further decrease of thickness with decrease of the mounting space and, in order to meet the demands, attempts have been made to implement a method for interposing the semiconductor chip between uncured encapsulating resin sheets and press-molding them under heat using the mold.
When the BGA package is mounted on a circuit board, the package has to experience a high-temperature process of 240 to 280 .degree.C., however, because the mounting process employs a step of vapor phase reflow, infrared reflow, dip soldering, or the like. If water permeates and diffuses into the encapsulating resin 9 to stay there the water will be vaporized suddenly in the high-temperature process, thus resulting in producing cracks in the encapsulating resin 9 in some cases. If these cracks reach the surface, moisture resistance reliability will not be assured. If the encapsulating resin 9 swells, it could pose another problem that the package cannot be mounted on the circuit board surface. In some cases, further problems would arise; for example, on the occasion of Ni--Au plating on wire circuits, delamination would occur at a solder resist film formed for the purposes of assuring heat resistance and preventing corrosion of the circuits themselves, or a break of the Au bonding wire 7 may occur on occasion.